1. Field of Invention.
This invention relates to improvements in semiconductor pressure sensors which use a piezo-resistance type gage.
2. Description of the Prior Art
FIGS. 1, 2 and 3 depict a conventional shearing gage made of a piezo-resistance type gage with FIG. 1 being a side sectional view, FIG. 2 being a plan view, and FIG. 3 being a detailed view of the main parts. A conventional shearing gage is disclosed, for example, in U.S. Pat. No. 3,213,681 and Japanese Patent No. 50049/1982.
In FIGS. 1,2 and 3, substrate 1 comprising a semiconductor has a diaphragm 4 mounted thereon for receiving a pressure P to be measured. Diaphragm 2 comprises a semiconductor single crystal having a crystal face (100). In this case, the diaphragm consists of a silicon semiconductor. A standard chamber 21 is provided on diaphragm 2 and receives a standard pressure Ps therein. A shearing gage 3 provided on diaphragm 2 at &lt;100&gt; axially, consists of a semiconductor with a p-type impurity, and is formed using ion implantation, thermal diffusion, etc. A surface of gage 3 is protected by a passivation film 31, as shown in FIG. 3, which consists of a silicon oxide (SiO.sub.2) film 311 and a nitride (Si.sub.3 N.sub.4) film 312, and an output is extracted by an electrode 32 consisting of an aluminum material.
As shown in FIG. 4, gage 3 has a gage body 321 connected to voltage feed terminals 322 and output terminals 323 through low resistance diffusion layers 33 consisting of p+type material. The voltage feed terminals 322 are provided on both ends of gage body 321 and supplied with a constant voltage Vs. The output terminals 323 are provided on gage body 321 orthogonal to the voltage feed terminals 322. A conductor part 34 is formed on the passivation film 31 opposite to shearing gage 3 and is kept at a predetermined potential E. The conductor part 34 comprises an n-type polycrystalline silicon. The conductor part 34 is kept at a constant potential by an external device (not shown).
In the above device, when a measuring pressure Pm is applied to diaphragm 1, an output voltage Eout corresponding to the measuring pressure Pm is obtained from output terminals 323.
Passivation film 31, comprising silicon oxide film 311 and nitride film 312, is formed for shearing gage 3 through thermal diffusion of an impurity, such as boron or the like, from the surface of diaphragm 2. Accordingly, silicon oxide film 311 contains a substantial amount of a moving ion and a surface charge. Thus, a surface field effect is brought to gage 3. When these moving ions and surface charges are subjected to a secular change, a characteristic of the shearing gage changes, and zero offset and drift of the sensor output thus arises.
Since conductor part 34 is kept at a constant potential, in case an impurity ion A in an external environment of the apparatus sticks on the surface of the apparatus, as shown in FIG. 5, it is ion A, having stuck on a portion opposite to gage 3 on an outer surface of the passivation film 31, that exerts an infuence on gage 3, and ion A sticks on conductor part 34 made of n-type polycrystalline silicon. A quantity of the sticking ion A changes as time passes, and the changing value comes out as drifting output. It is thus necessary to minimize that influence to insure a stable pressure sensor. Conductor part 34 is made of n-type polycrystalline silicon and is a conductor and is kept at a constant potential. Thus, the potential of the apparatus surface is kept at a constant potential despite existence of impurity ion A. Thus, resistance value and output of gage 3 will not change due to influence of impurity ion A.
Accordingly, zero offset, dispersion of output at every sensor, drift of output, etc, will be minimized, and a semiconductor pressure sensor which is inexpensive, high in reliability and precision is obtained.
However, in the conventional device, since the conductor part 34 is on top of nitride film 312, a separate process is needed to get out ion in the oxide film on the piezoresistance element.
Furthermore, in the prior device, the forming temperature of the passivation film 31 is high (the silicon oxide film 311 being formed at 1,100.degree. C., and the nitride film 312 being formed at about 800.degree. C.). Therefore, residual stress works on a portion of the shearing gage 3 in an operating state under ordinary temperature. The stress is produced according to the difference in coefficients of thermal expansion between the passivation film 31 and the silicon. This causes zero offset. Also, a quantity of stress disperses on a slight difference of the manufacturing process and produces drift with passage of time. This produces deterioration of reliability.
For manufacturing shearing gage 3 through photolithography, it is necessary that thermal diffusion be carried out to form the gage 3. Also, thermal diffusion must be carried out to form the low resistance diffusion layer 33 for lead wire. The thermal diffusion is necessary to be carried out two or more times. Thus, the oxide film surface may drop in level by a drop B.
On the other hand, since passivation film 31 is different in expansion coefficient from diaphragm 2, a film stress is produced when ordinary temperature state occurs after the thermal diffusion. In this case, where passivation film 31 surface has dropped in level, local stress is produced to cause zero offset and drift of the sensor output.
Assuming that tensile stresses in directions x and y arising on the surface of diaphragm 2 are .sigma..sub.x and .sigma..sub.y, respectively, .sigma..sub.x =.sigma..sub.y generally barring an end of diaphragm 2. Accordingly, the output of gage 3 becomes zero since shearing stress .tau..sub.xy =(.sigma..sub.x -.sigma..sub.y)/2 is zero.
However, if there exists a drop in the level of the film surface and a portion is uneven in film thickness due to photolithography or other reasons, then .sigma..sub.x .noteq..tau..sub.y, the shearing stress .tau..sub.xy =(.sigma..sub.x -.sigma..sub.y)/2 is not zero, and thus the output of the gage 3 does not hold zero. The status is shown in FIG. 6. A portion indicated by an arrow X is that in which a local shearing stress arises in concentration.